An alloy A is formed by mixing copper and nickel in the ratio 1:2,by weight and another alloy B is formed by mixing nickel and zinc in the ratio 3:4, by weight. If a third alloy C is formed by mixing the alloy Aand B IN CERTAIN ratio such that the ratio of copper and zinc in C is 5:6,by weight,then what fraction by weight of the alloy is nickel.